Piston rings have recently become used in increasingly severer environment, due to higher power of engines, higher combustion temperatures and larger load for meeting exhaust gas regulations, the use of low-viscosity lubricating oils, the diversification of fuels such as bioethanol, higher fuel injection pressure, etc. Even piston rings having hard chromium nitride (CrN) coatings formed by ion plating, which are now considered as having best scuffing resistance and wear resistance, may fail to exhibit sufficient performance due to the peeling of coatings by cracking and chipping. Accordingly, improvement in scuffing resistance, wear resistance and peeling resistance has been more strongly demanded than ever.
Because the chromium nitride formed by ion plating generally suffers chipping despite hardness, various improvements such as the controlling of crystal orientations, structures and porosity, the addition of a third element, the lamination of coatings, etc., have been made so far.
With respect to the addition of a third element to chromium nitride, to improve the toughness of CrN, JP 6-265023 A teaches the addition of oxygen (O), and JP 6-300130 A teaches the addition of carbon (C), both for solid solution. In addition, WO 2008/059791 discloses a chromium nitride coating having excellent wear resistance and cracking resistance, the coating having a columnar structure toward a coating surface; the concentration of carbon being 4-8% by weight per the total amount of main components of chromium, nitrogen and carbon; and the coating having Vickers hardness of 1600 or more, and fracture toughness of 3 MPa·m1/2 or more.
With respect to the lamination of coating layers, for example, to solve the problem of the chipping-like peeling of a coating from an outer peripheral surface of a piston ring due to pitting fatigue, JP 8-312779 A discloses a coating formed by alternately laminating layers of columnar CrN crystals extending from the base material surface toward the coating surface and CrN crystal layers having flat fracture surface, or formed by alternately laminating layers having porosity of 0-0.5% by volume and layers having porosity of 1.5-20% by volume. From the same aspect, for example, JP 2005-187859 A discloses a hard, thick composite nitride coating of CrSiN and TiSiN; which comprises stress relaxation layers having a non-columnar crystal structure, which are sandwiched by hard layers of composite nitride columnar crystals with constant intervals and thickness to have reduced internal stress and high adhesion; the composite nitride coating being formed by alternately repeating high bias voltage for forming columnar crystals and low bias voltage for forming non-columnar crystals with predetermined intervals.
In both JP 8-312779 A and JP 2005-187859 A, layers of the same nitride with different structures (columnar structure layers and non-columnar structure layers, or porous layers and dense layers) are laminated. JP 2005-82822 A discloses a laminate coating comprising metal layers as the stress relaxation layers in JP 2005-187859 A, namely, a laminate coating having different compositions. However, the metal layers are disadvantageous in scuffing resistance, despite better stress relaxation than in the composite nitride layer of JP 2005-187859 A.
JP 2007-278314 A discloses a piston ring having a laminate coating obtained by alternately laminating compound layers comprising titanium, carbon and nitrogen, and compound layers comprising chromium, carbon and nitrogen. To provide the coating with excellent wear resistance without chipping when used as a top ring for diesel engines, the amount of carbon dissolved in the coating and the porosity of the coating are determined to have fracture toughness of about 3 MPa·m1/2 or more and hardness Hv of 1700 or more, which have close relations to the performance of the coating.
However, even the laminated coatings described above are not sufficient for piston rings used in extremely severe environment. Cracking, chipping and peeling of the hard coatings due to fatigue are actually not completely overcome.